There are many parties who would benefit from early detection and warning of degradation of tire performance or of imminent failure of a tire. For example, in passenger car tires, off-road tires, in truck or bus radial tires, the consumer, those in industry and government are all interested in improving safety and operational efficiency of vehicles with tires.
Some interesting statistics have been found through various industry studies (Cycloid National Tire Survey, for example) to show various aspects of tire data. For example:                1. Almost 25% of the total truck and trailer tire inventory experiences problems each year. Over 80% of all tire problems experienced by fleets are a direct result of improper inflation.        2. A 10-degree drop in air temperature will reduce tire pressure by 1 psi. The difference between summer and winter tire pressures can be as much as 8 psi, causing a fuel penalty.        3. For truck tires, a 10 psi drop in tire pressure will increase tire rolling resistance by 2%, resulting in loss of fuel economy and increase in operating temperature.        4. Tires under-inflated by 15% will cause fuel penalties of about 2.5%; 30% under-inflation will increase fuel consumption by about 5%.        5. Tire costs are the second highest operating cost after fuel. Driver costs have fallen to third in operating expenses.        6. An industry average of 20% of the annual maintenance budget is spent on tires and tire management programs.        7. Running tires at 20–30% under-inflation can reduce tire life by up to 50%, cause increased tire running temperatures, lead to premature tire removal, and also reduce the life of the casing, which affects the ability to retread.        8. Surveys indicate that consistent proper tire inflation for the trucking industry could increase fleet wear by 17%.        9. A 100 psi tire run under-inflated at 80 psi will lose 15–20% of its life, and damage its further life as a retread candidate.        10. Tire abuse causes rubber debris on the road. Almost all debris comes from dual tires, primarily from the inner tire.        11. An under-inflated condition, which weakens the sidewall cables, causes failure.        12. The average cost for an emergency roadside assistance call is about $775.00.        13. Retreads use 7 gallons of oil to re-manufacture a tire versus 22 gallons for a new tire.        14. Proper tire inflation could save 25,000 lives each year.        
These studies show the importance of good tire management and the impact on cost, safety, and environment. Tire failures have produced some terrible consequences in terms of the loss of life and property. Failure of the user to maintain the tires is an important factor. Air pressure may not always be considered to be a continuous maintenance process by the operators of vehicles. Thus, operating tires with low air pressures often does not manifest itself immediately, but rather, over a period of time.
Truck tires operate with greater load pressures and sidewall stress than a normal automobile tire. The temperature of the tire created by the friction of running up and down the highway is also much higher in a truck. All of these factors add to the importance to practice regular tire maintenance for both original rubber tires and recap tires. Both are susceptible to the same factors, and their lives will be a reflection of the maintenance they receive. In vehicles with high daily mileage, tire pressure must be checked on a daily basis considering the risks involved in picking up a nail, bolt, wire, suffering a sidewall cut, etc. Caps should be kept on the valve stems. Front axle or steering tires are usually considered to be the most important of the 18 tires on a large rig. A radial tire cannot safely be run if low on air pressure, particularly on the steering axle. With greater loads bearing upon the tires, higher speeds create higher operating temperatures, making regular maintenance of tires (including balancing and alignment) more critical than ever.
Many prior art patents focus on measurement of tire pressure, indicating that tire pressure is perceived as an important parameter to measure. Those in the transportation industry are aware that their vehicles do indeed get better fuel mileage and longer wear if tires are properly inflated. In some cases, temperature of the air inside the tire or temperature of the tire and tread are also measured because laws of physics relate pressure and temperature. Charles Law (V/T=constant), states that gas expands by the same fraction with each degree that it's temperature rises. Thus, the ratio of volume (V) to temperature (T) does not change if the pressure remains constant. Boyle's Law (PV=constant) states that the product of pressure (P) multiplied by volume (V) remains constant if there is no change in temperature or if the number of particles in a container remains constant.
The universal gas law “PV=nRT” combines the aforementioned Boyle's and Charle's law with Avogadro's law (which states that an equal volume of all gases have the same amount of particles given the same pressure and temperature). In this law P is the gas pressure, V is the volume, n is the number of moles of gas, R is the universal gas constant with a value of 8.314 joules per Kelvin per mole and T is its absolute temperature. This universal gas law states that temperature can, for example, be doubled in three ways; (1) Gas volume can be doubled from its original volume, (2) The pressure of gas can be doubled, (3) The amount of moles of gas can be halved. It can also be easily seen that a loss of air molecules can result in an increase in temperature or loss of pressure. With no air molecule loss, as temperature rises inside a tire, the pressure may also rise, thus “low” pressure is not always an indication of tire health.
Thus, knowing both temperature and pressure of a gas in a fixed vessel, such as a tire, allows one to normalize the pressure at a standard temperature such as 20 degc. This would yield a meaningful and normalized pressure measurement, having been corrected by air temperature.
It is well understood by tire manufacturers that the actual main failure mechanism in radial tires is the separation (delaminating) of the inner liner from the inner wall of the tire due “directly” to high temperature, not to low pressure. Thus, and for other reasons noted herein, it can be simpler, more effective, cheaper and more reliable to measure temperature instead of pressure-only or pressure-corrected-by-temperature. If a tire maintains normal temperature in operation, then the tire is not facing imminent self-destruction or excessive wear, and is not consuming extra fuel; at least not to any greater extent than would be inferred by monitoring pressure inside the tire air chamber.
Many companies recognize the value of monitoring tire temperature. For example, Goodyear® provides a “Temperature Prediction Model” that uses peak tire temperature—a major factor in tire failures—instead of the traditional kilometer-per-hour/ton-mile-per-hour formula to select and manage tires in service. The model provides a forecast of temperature at the hot spot of the tires, based on a myriad of variables to maximize tire life by knowing when the tires are entering the heat danger zone and taking steps to keep them out of that zone by putting the vehicle on a different route, performing maintenance, or some other activity. It anticipates the impact of route configuration, ambient temperature, expected weather forecast, route description, timing, distance and speed among other variables in actual tire model temperature and pressure prediction.
Onboard microcomputers, attached to the interior of a tire, to monitor various parameters (temperature, pressure and load) is the goal of many tire companies. A system to reduce downtime, extend tire life, eliminate field checks, increase safety and save money is the goal.
Most manufacturers' systems operate on basically the same level. Monitoring chips are located inside the air chambers of the tire and monitor operating parameters. That information is then transferred to an onboard receiver mounted somewhere on the vehicle and/or is sent to monitors in the fleet office.
The theory behind tire chip technology would be to provide operators with a display to see, on a real-time basis, which tires are getting hot, and adjust routes to avoid excessive heat which is the tire's worst enemy.
Some companies have a system that is slightly different in that there is no onboard receiver. The vehicles must pass through a fixed gate reader where the tire chips are scanned, and the information is then transferred to the fleet office.
Whereas nothing needs to be done with the tire if all monitored parameters are satisfactory. If there is a problem with a tire, a computer is alerted to what the problem is—be it low pressure, high temperatures, etc. such that the customer can fix the problem before it cascades into something much larger and more expensive.
External monitoring would show indications at various levels; (1) Potential problems that may arise, (2) Problems requiring some action, and (3) Emergency situations requiring stopping the vehicle and having it serviced immediately.
One problem with a technology located within a tire is that it has to withstand a lot of punishment. Moving earth out of a quarry with a 240-ton truck, an 18-wheel rig with a heavy load, or other road conditions submit a computer chip to a great deal of stress. A monitoring technology must also last anywhere from two to five years with no maintenance. System failures, where the sensor is inside a tire, would require the tire to be removed and, thus, be costly.
Many manufactures are concentrating on monitoring of tire pressure, believing that a substantial number of tire problems, equipment downtime and excessive tire wear are due to under inflation. Thus, their goal is to place a gauge inside a tire and monitor a vehicle's tire pressure. As previously stated, tire pressure is not a sole determinant in finding problems. As aforementioned in universal gas law (PV=nRT) it can be easily seen that if the temperature increases, for example doubles, then the pressure would double. Thus, if a tire started to lose air molecules, causing it to deflate, temperature would get hotter. The higher temperature could result in pressure increasing and may not reveal that there is a problem if monitoring pressure alone.
Irrespective of avoiding catastrophic failure due to liner separation, the fuel inefficiencies and increased tire wear due to low-pressure cause heat to be generated in the tire. Thus, measurement of tire temperature can provide early detection of catastrophic failure in addition to improved operational efficiencies because the degradation mechanism is the same; namely, increased road friction from the larger “footprint” due to the tire shape flattening as a result of being under-filled while under load. It is also important to note that detecting low pressure will not identify impending tire failure, whereas detecting tire temperature will identify impending tire failure, even if measured through the rim.
Measuring tire temperature itself is problematic in that tires flex, raising engineering hurdles such as “how to attach a sensor to a flexing object?” It is also difficult technologically to send the pressure or temperature data from a device within the tire cavity.
Users of tire monitoring systems would strongly prefer that the mounting and maintenance of a tire monitoring system not require removing the tire from the rim either for installation or maintenance of the monitoring system, over the life of the tire, which can be years.
Thus, it is desirable to place the sensor on the rim inside the tire cavity or outside the tire cavity, instead of in, or on, the tire itself. An alternative location is in a hole in the rim that communicates to the inside of the tire air chamber. If a sensor is on the tire, tires are periodically replaced, implying that the sensor must be discarded with the old tire or recovered for placement in the new tire.
The present invention solves the aforementioned problems by providing the benefit of a sensor mountable to the exterior of the rim where it is easy to maintain. For example, if sensor batteries need to be changed, then they are accessible on the outside of the tire rim.